MTTF (Mean Time to Failure)

MTTF (Mean Time to Failure) is the average length of time something works before it fails for good. It is used for items that are replaced rather than repaired, such as a disk drive, a power supply, or a fan.

Why it matters

MTTF helps you plan spares, replacement cycles, and redundancy. If a component has a long MTTF but you run thousands of them, some will still fail every month, so large fleets need to expect failures as routine. It is a statistic over a population, not a prediction for any single unit.

Example

You monitor 500 drives for 10,000 hours each, a total of 5,000,000 device-hours, and observe 10 failures:

MTTF = total operating time / number of failures
     = 5,000,000 / 10
     = 500,000 hours

Do not read this as "each drive lasts 57 years". Failure rates change over a device's life, with early failures, a stable middle period, and wear-out at the end.

Related terms: MTBF applies to repairable systems and counts time between failures, and MTTR is how long recovery takes.

Common mistakes

Using MTTF and MTBF interchangeably. MTTF is for parts you replace; MTBF is for systems you repair and return to service.
Treating a vendor-published figure as a guarantee of lifespan.
Ignoring that software failures are not governed by wear-out and need different measures.

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